Modern wind energy installations are designed for variable rotation speeds. This means that the wind rotor revolves at a rotation speed which is governed by the load, and that the generator produces electrical power at a correspondingly variable frequency. In order to allow this variable-frequency electrical power to be fed into a fixed-frequency supply network, a converter is provided, and is connected to the generator. Converters such as these normally comprise two inverters, one of which acts as a rectifier, and which are connected via a DC voltage or direct-current intermediate circuit. In this case, one of the inverters is connected to the network and has the network frequency applied to it during operation (network-side inverter), while the other inverter (machine-side inverter) is connected to the generator, with the frequency at it being governed inter alia by the rotation speed of the wind rotor. Converters such as these may be in the form of full converters or partial converters, the latter in particular in combination with a double-fed asynchronous machine. It has been found that operation in the region of the synchronous rotation speed (synchronization point) results in problems. In the region around the synchronization point, the frequency at the machine-side inverter is very low, and, in the extreme, may even become direct current when precisely at the synchronization point. As a result of the low frequencies, phases with a severe thermal load and phases with a light thermal load no longer alternate sufficiently quickly, as a result of which the switching elements which are in each case switched on, in particular, are subject to an increased thermal load. This adversely affects the life of the switching elements, and can lead to them failing. During operation in the region of the synchronization point, when the frequency acting on the machine-side inverter can accordingly reach very low values, which may be down to zero, the maximum permissible current load on the switching elements is reduced. This can lead to the maximum permissible current load being reduced by up to half. However, from the mechanical point of view, a reduction in the maximum permissible current actually means that the maximum permissible torque acting on the wind rotor is limited to a correspondingly greater extent. In consequence, the controllable rotation-speed range around the synchronization point is restricted by the more greatly limited, that is to say reduced, torque. This runs contrary to the concept of the variable rotation-speed wind energy installation and prevents the use of rotation-speed/torque characteristics with high torques beyond the synchronization point, as is required, for example, for low-noise operation at a reduced rotation speed or for rapidly passing through rotation-speed ranges in order to avoid tower resonances.
In the case of power converters, in particular for AC motors, it is admittedly known for a potential shift to be carried out for individual phases, in such a way that they are clamped at an intermediate-circuit potential and the pulsing can therefore be suspended at times (DE-A-102 43 602). In this case, the phase with the greatest magnitude is clamped at one of the intermediate-circuit potentials, as a result of which the pulsing can be suspended. Furthermore, the phase with the second greatest potential magnitude is clamped to one intermediate-circuit potential or the other alternately, depending on the phase angle. The latter phase is therefore operated at a reduced switching frequency. This is sufficient for driving motors, but, in the case of variable rotation-speed wind energy installations, can lead to the pulse repetition frequency of the relevant switching elements falling excessively during operation in the region of the synchronous rotation speed and therefore still increasing the thermal load, particularly on the switching elements for this phase. In fact, this known power converter is therefore not actually suitable for use with variable rotation-speed wind energy installations.